The construction and principle of the free piston combustion engine is disclosed in FI-A-80760 and in U.S. Pat. No.-A-4,992,301. Worth mentioning as further references to the conventional technique are U.S. Pat. No.-A-3,089,305, U.S. Pat. No.-A-3,995,974 and U.S. Pat. No.-A-4,097,198.
One of the problems associated with the development of free piston combustion engines has arisen from starting of the engine. According to the method described in U.S. Pat. No.-A-3,995,974, starting is performed using the same hydraulic cylinder-operated apparatus that after starting is used for power output from the engine, and the valve elements, which are included in the hydraulic circuit between the hydraulic accumulator for the starting pressure and the cylinders of the hydraulic piston/cylinder device, are controlled to switch their position so as to cause the opposite sides of the hydraulic piston/cylinder device to be alternately pressurized, while simultaneously allowing the cylinder space opposite to that being pressurized to be connected via a return line to the hydraulic reservoir. Such an arrangement achieves a reciprocating motion in a simple and useful manner without a dedicated hydraulic power source, whereby the engine construction is simplified, the weight is simultaneously reduced and the price minimized.
There remains, however, the need for starting the reciprocating motion of the free piston unit with the help of sufficient inertial energy before the combustion process can be initiated.
Therefore, in many appliances it would be advantageous to be able to use an essentially high pressure as the starting pressure than the pressure available in the pressure accumulator provided between the hydraulic piston/cylinder device and the actuator itself. This is not possible, however, if the pressure of the hydraulic cylinders is allowed also during starting to be imposed over the pressure-driven check valves to the pressure accumulator of the load and the pressure-regulating valve; and further, to the flywheel engine, which is driven by the pressure of the hydraulic cylinders during the normal running of the engine as described in U.S. Pat. No.-A-3,995,974.
A special problem with the last mentioned prior art proposal is that the hydraulic circuit to the load is arranged to be cut off during the starting phase by a separate clutch valve. The clutch valves presently available are, however, too slow to make the necessary ON and OFF switching with the high flow rates and pressures in question.